Conventionally, in W-CDMA (Wideband-Code Division Multiple Access), a base-station control device (a radio network controller (RNC)) controls packet transmission between a base station and a mobile station. However, due to degradation of line quality or the like, loss of a packet to be transmitted (packet loss) may sometimes occur between the RNC and the mobile station. Especially, as the base station and the mobile station are connected by a radio link, packet loss may occur frequently depending on a radio wave condition. To cover such packet loss, a W-CDMA wireless communication system has a retransmission function of retransmitting a transmitted packet. Namely, the W-CDMA wireless communication system normally remedies packet loss by retransmission control in the RLC (Radio Link Control) layer, thereby ensuring stable data communication between a mobile station and its higher-level device.    Patent document 1: Japanese Laid-open Patent Publication No. 2001-160824    Patent document 2: Japanese Laid-open Patent Publication No. 2001-358763
However, a line bandwidth between an RNC and a mobile station is the sum of an effective rate used in normal packet communication and a retransmission rate used in retransmission of a packet; therefore, when a ratio of retransmission in the RLC layer is increased with the degradation of line quality or the like, the proportion of an effective rate to the line bandwidth decreases. As a result, line usage efficiency between the RNC and the mobile station is reduced. When there is enough line bandwidth, even if the line usage efficiency is reduced by a certain extent, the RNC can ensure stable data communication by increasing the effective rate. However, when the line bandwidth is tight, such as when there are many mobile stations with high packet retransmission ratios, there is concern that ensuring of stability in data communication becomes difficult due to a decrease in line usage efficiency. The concern is particularly prominent when a line or device is in a congestion state. Incidentally, such a problem may occur in not only a W-CDMA wireless communication system but also an LTE (Long Term Evolution) wireless communication system that controls retransmission between a base station (evolved Node B (eNB)) and a mobile station (User Equipment (UE)).